The present invention generally relates to a semiconductor device and a method for fabricating the same. More particularly, the present invention relates to a semiconductor device, including a capacitor, which can suppress leakage current sufficiently, and to a method for fabricating the device.
As a semiconductor device has been downsized, it has become more and more necessary to improve its performance by taking full advantage of the properties of materials for the device. For example, if the number of devices integrated on a single chip should be increased for a dynamic or ferroelectric random access memory (i.e., DRAM or FeRAM), which stores information thereon using transistors and capacitors in combination, then not only the transistors but also capacitors should be miniaturized.
As for a capacitor, its capacitance should be at least equal to a predetermined value, and preferably more, even if its area on the chip has been reduced as a result of downsizing. This is because noise and soft error might seriously affect the performance of an overly downsized capacitor. A capacitor has often been made of a multi-layered dielectric film, e.g., an ONO film in which silicon dioxide and silicon nitride layers are stacked one upon the other. However, since a higher capacitance should be now attained from a capacitor of a reduced size, alternative materials with dielectric constants higher than that of ONO have been looked for. A film made of any of those high-dielectric-constant materials will be herein called a xe2x80x9cfunctional material filmxe2x80x9d.
Examples of proposed functional material films with high dielectric constants include a perovskite oxide dielectric thin film of tantalum pentoxide (Ta2O5), strontium titanate (SrTiO3, which will be abbreviated as xe2x80x9cSTOxe2x80x9d), barium titanate (BaTiO3, which will be abbreviated as xe2x80x9cSTOxe2x80x9d) or barium strontium titanate (BaxSr1-xTiO3, which will be abbreviated as xe2x80x9cBSTxe2x80x9d).
However, if one of these functional materials is used for a capacitive insulating film of a capacitor, then the electrode of the capacitor cannot be made of polysilicon anymore. This is because those functional material films are often formed within an oxidizing ambient. That is to say, if polysilicon is exposed to the oxidizing ambient, then its surface is oxidized to form a silicon dioxide film with a relatively low dielectric constant. In such a situation, even if the capacitive insulating film is made of a material with a high dielectric constant, the effective quantity of storable charge decreases due to the existence of the silicon dioxide film with a low dielectric constant.
Thus, when one of those functional materials is used, the electrode is made of a noble metal such as platinum (Pt), ruthenium (Ru) or iridium (Ir).
An exemplary structure with a capacitor made of Ta2O5 is disclosed in Japanese Journal of Applied Physics, 37, (1998), pp. 1336-1339, while an exemplary structure with a capacitor made of BST is disclosed in Technical Digest of International Electron Device and Materials (1998), pp. 253-256.
Among these noble metal materials, Ru is particularly promising as an alternative electrode material. This is because ruthenium dioxide (RuO2), one of its oxides, is a conductor and because ruthenium tetroxide (RuO4), another oxide thereof, has a high vapor pressure at a low temperature and can be shaped by dry etching.
However, the higher the dielectric constant of such a functional material film, the higher the density of leakage current flowing between electrodes when a voltage is applied thereto. The increase in density of leakage current adversely decreases the quantity of stored charge with time. As a result, the charge is storable by a DRAM for a shorter period of time.
Next, a technique disclosed in Articles for the 54th Symposium on Semiconductor and Integrated Circuit Technology, pp. 12 to 17, will be briefly described. According to the technique disclosed in this article, a lower electrode of ruthenium is formed to a thickness of 100 nm on a thermal oxide film that has been formed on a silicon substrate, and then annealed for 30 seconds within a nitrogen ambient at 700xc2x0 C.
Thereafter, a Ta2O5 film is formed to a thickness of 24 nm on the lower electrode by a chemical vapor deposition (CVD) process. In such a state, the composition of the Ta2O5 film deviates from that defined by stoichiometry and the Ta2O5 film contains Ta excessively. A very large amount of leakage current will flow as it is. To avoid such an unfavorable situation, the Ta2O5 film is annealed for an hour within an oxygen ambient at 550xc2x0 C. As a result of this annealing, the density of leakage current will be 1xc3x9710xe2x88x928 A/cm2 for a field intensity of 1 MV/cm, and the dielectric constant will be 30.
According to another technique disclosed in the above-identified document, oxygen may also be supplemented by performing oxygen plasma annealing at 300xc2x0 C. for 10 minutes. As a result of this oxygen plasma annealing, the dielectric constant will also be about 30 and the density of leakage current will be 1xc3x9710xe2x88x928 A/cm2 for a field intensity of 1.5 MV/cm.
The prior art capacitor has a low leakage current value after having been annealed for an hour within an oxygen ambient at 550xc2x0 C. or for 10 minutes within oxygen plasma at 300xc2x0 C. However, at this point in time, the Ta2O5 film has not been crystallized yet and has a dielectric constant as low as about 30, which is not so much greater compared to a conventional capacitor using an oxynitride film and a polysilicon electrode with a roughened surface. Thus, it is not advantageous to use the ruthenium electrode and the Ta2O film.
To increase the dielectric constant, the Ta2O5 film should be crystallized by being annealed at about 700xc2x0 C. According to the technique disclosed in the above-identified article, the Ta2O5 film is crystallized by being annealed for 60 seconds within a nitrogen ambient at 750xc2x0 C. After the Ta2O5 film has been crystallized, the electrical characteristics of the capacitor improve to a certain degree. Specifically, a sample that has been annealed for an hour within an oxygen ambient at 550xc2x0 C. has a dielectric constant of about 60 and a leakage current density of 1xc3x9710xe2x88x925 A/cm2 for a field intensity of 1 Mv/cm.
A sample that has been annealed with oxygen plasma to supplement oxygen thereto has a dielectric constant of about 60 and a leakage current density of 1xc3x9710xe2x88x928 A/cm2 for a field intensity of 1 MV/cm or 1xc3x9710xe2x88x926 A/cm2 for a field intensity of xe2x88x921 MV/cm. As can be seen, in supplementing oxygen, the oxygen plasma annealing attains a lower leakage current density compared to the annealing within the oxygen ambient. However, even the lower leakage current density is far from being sufficiently low.
It is therefore an object of the present invention to reduce the leakage current sufficiently when an electrode for a capacitor is made of a noble metal.
A more specific object of the present invention is to reduce the leakage current sufficiently for a capacitor including a functional material film and a noble metal electrode.
To achieve these objects, when a conductor film for an electrode, e.g., a ruthenium film, is crystallized, the crystal grains of ruthenium are grown to have stepped surfaces according to the present invention.
Specifically, a first inventive semiconductor device includes an electrode, which is formed over a substrate and contains ruthenium. Crystal grains of ruthenium contained in the electrode have stepped surfaces.
In the first semiconductor device, the crystal grains of ruthenium, which is a material for the electrode, have stepped surfaces. That is to say, since the surface area of each crystal grain is greater, the apparent dielectric constant increases compared to normal crystal grains with no stepped surfaces. In addition, a plane linking adjacent crystal grains together forms an obtuse angle with the surface of each adjacent crystal grain as will be described later. Thus, an electric field is less likely to be concentrated on the interface between these crystal grains, and therefore, the leakage current decreases. Accordingly, by applying the inventive electrode to a capacitor, including a capacitive insulating film made of a functional material with a high dielectric constant, a semiconductor device with reduced leakage current can be obtained.
A second inventive semiconductor device includes: a lower electrode formed over a substrate; a capacitive insulating film formed on the lower electrode; and an upper electrode formed on the capacitive insulating film. The lower electrode includes crystal grains having stepped surfaces.
In the second semiconductor device, the crystal grains of a material for the lower electrode have stepped surfaces. That is to say, since the surface area of each crystal grain is greater, the apparent dielectric constant increases compared to normal crystal grains with no stepped surfaces. In addition, a plane linking adjacent crystal grains together forms an obtuse angle with the surface of each adjacent crystal grain. Thus, an electric field is less likely to be concentrated on the interface between these crystal grains, and therefore, the leakage current decreases. Accordingly, by applying the inventive electrode to a capacitor including a capacitive insulating film made of a functional material with a high dielectric constant, a semiconductor device with reduced leakage current can be obtained.
In one embodiment of the present invention, the lower electrode preferably contains ruthenium. In such an embodiment, the surfaces of ruthenium crystal grains are likely to be stepped.
A first inventive method for fabricating a semiconductor device includes the steps of: a) forming a conductor film containing ruthenium over a substrate; b) forming an electrode out of the conductor film by patterning the conductor film into a predetermined shape; and c) annealing the electrode and the substrate within a non-oxidizing ambient, thereby shaping the surfaces of crystal grains of ruthenium contained in the conductor film into stepped ones.
According to the first method, the surfaces of ruthenium crystal grains for the conductor film are shaped into stepped ones by conducting an annealing process within a non-oxidizing ambient. Thus, the first inventive semiconductor device is obtained by the first method.
In one embodiment of the present invention, the non-oxidizing ambient preferably contains hydrogen. Then, the surfaces of ruthenium crystal grains can be shaped into fine steps by doing so.
A second inventive method for fabricating a semiconductor device includes the steps of: a) forming a conductor film containing ruthenium over a substrate; b) forming a lower electrode out of the conductor film by patterning the conductor film into a predetermined shape; c) annealing the electrode and the substrate within a non-oxidizing ambient, thereby shaping the surfaces of crystal grains of ruthenium contained in the conductor film into stepped ones; and d) forming a capacitive insulating film on the lower electrode.
According to the second method, the surfaces of ruthenium crystal grains in the lower electrode are shaped into stepped ones by conducting an annealing process within a non-oxidizing ambient. Thus, the second inventive semiconductor device is obtained by the second method.
In one embodiment of the present invention, the non-oxidizing ambient preferably contains hydrogen.
In another embodiment of the present invention, the capacitive insulating film is preferably made of tantalum pentoxide, strontium titanate, barium titanate or barium strontium titanate. In such an embodiment, since the capacitive insulating film is made of a material with a high dielectric constant, a desired capacitance (or desired quantity of charge stored) is ensured even if the semiconductor device including the capacitor is downsized.
In still another embodiment, the step c) is preferably performed before the step d). In such an embodiment, the conductor film can be annealed within a deposition system that will be used for forming the capacitive insulating film, thus simplifying the process.
In yet another embodiment, the step c) is preferably performed after the step b). In such an embodiment, the lower electrode is patterned before the crystal grains have grown so densely through annealing that the electrode is hard to shape. That is to say, the patterning process is easier to carry out.
In still another embodiment, the step a) preferably includes the step of forming the conductor film in the shape of a bottomed cylinder. In such an embodiment, the surface area of the lower electrode is greater, and therefore, the capacitance of the capacitor increases just as intended.